?????zooming in, it does look like the holes are "hex-ish"
im with lub, i would have on hand the plugs that are SUPPOSED to be in there.
side thought, there must be a tool that takes advantage of that hole in the center, like visegrips, with a pin for the center, to keep the grips from comming off. ya got me???

maybe take your pics to a snap on tool, or mac, and see what they say. they might have a tool??